Shell has additional strength far superior to calf due to the shells cross growth structure it is similar to extra warp and weft weaving on shirts. The fact is two people pulling at calf skin will be able to create a tear, shell is different since the two people will not be able to tear the hide. Only other skin close to shell is camel hide and it is said to be superior to shell in comfort not strength or shine.

How are these rare?
I suppose these are soft toe like The Bureau Belfast has, and also a flex sole.
http://www.ebay.com/itm/Rare-Alden-Plain-Toe-Blucher-Shell-Cordovan-Color-8-Size-7-5-/330962826671?pt=US_Men_s_Shoes&hash=item4d0eebf5af